Standards for emissions of particulate in flue gases issuing from coal fired electrical power station stacks are becoming increasingly more stringent. Current air quality standards require that more than 99% of the fly ash produced by burning coal be removed prior to discharge of the combustion gases from the stack. Thus, the efficiency of particulate collection must increase in proportion to the ash content of the coal. In addition, in an effort to reduce the emission of certain gaseous pollutants, particularly the sulfur oxides, it has become increasingly necessary to use low sulfur coal in electrical power generating plants.
The electrostatic precipitator is a commonly used device for the removal of particulate matter from power station stack gases. Because the size of an electrostatic precipitator is determined by the efficiency of fly ash removal required, an increase in required fly ash collection efficiency required a corresponding increase in equipment size and cost. Moreover, because fly ash resistivity tends to be inversely related to the level of combustible sulfur in the coal burned, the use of low sulfur coals to directly reduce gaseous sulfur oxide emissions, produces highly resistive dusts. It has been demonstrated that the size of the electrostatic precipitator necessary to achieve a given level of collection efficiency increases with increasing electrical resistivity of the fly ash. The use of low sulfur coals therefore further increases the size and cost of the precipitator.
Recently, high-intensity ionizers have been developed in which a unique electrode geometry produces a stable high-intensity corona discharge through which the particulate-laden gas is passed. The ionized flue gases produced charge the particulate matter to a much higher level than is achievable with a conventional electrostatic precipitator. When the ionizer is followed with an electrostatic precipitator, the high particle charge results in a higher collection efficiency in the precipitator due to higher migration or particle drift velocity. In such a two-stage arrangement, the ionizer acts as the charging stage and the precipitator serves as the collecting stage.
Such high-intensity ionizers utilize a co-axial pair of electrodes to generate a high-intensity electric field expanding radially and axially with respect to the direction of gas flow. The anode in such an arrangement typically takes the form of a venturi diffuser through which the stack gases flow. The venturi diffuser typically includes an inlet, a throat section, a diffuser, and a metal or fiberglass expansion cone through which the gases flow immediately prior to entering the precipitator stage. The cathode may be a disk coaxially mounted within the venturi throat and is formed with a curved peripheral edge having a radius much smaller than the inner radius of the venturi diffuser. When a high voltage power supply is connected between the anode and cathode, a high-intensity corona discharge is established in the region between the arcuate periphery of the cathode disk and the surrounding tubular anode surface near the disk. Because the field is relatively narrow in the direction of gas flow, a high intensity electric field is achievable without prohibitive power requirements. The combination of the high gas stream velocity through the venturi and the high intensity transverse electric field through which the gas stream passes produces intense ionization and very high levels of charge on the particles and results in increased collection efficiency notwithstanding the high resistivity of the particulate as in the case of fly ash from low sulfur coal.
One of the problems which has been encountered in connection with co-axial high intensity ionizers of the type described above results from the detrimental build-up of charged particles on the anode wall near the corona discharge plane. Deposition of high resistivity particulate matter in this region results in the phenomena of back corona and excessive sparking with a resulting deterioration in the electrical field and degradation in particle charge. Prior attempts, see U.S. Pat. No. 4,093,430, issued June 6, 1978, to overcome this problem have involved "cleaning" the anode surface in the affected region to eliminate disturbances in the corona due to contaminant build-up on the outer electrode. This cleaning has been accomplished by injecting water or similar fluid onto the surface of the converging cone section of the venturi wall. U.S. Pat. No. 4,108,615 issued Aug. 22, 1978, discloses another approach to the deposition problem. Here the venturi diffuser has a vaned anode through which clean gas from an external source is introduced into the venturi throat section to form a clean gas protective barrier along the interior anode surface.
Another problem which has limited performance of the high-intensity ionizer/precipitator arrangement has been the formation of back corona at certain geometric interfaces within the ionizer assembly. Particularly, particle discharge, which causes back corona, was found to occur at the interface between the diffuser and fiberglass expansion cone of the venturi diffuser and at the outlet of the expansion cone. There are several possible causes for this back corona including a buildup of particulate matter and/or the existence of sharp edges and small protuberances at the above-described locations within the ionizer, the change in dielectric properties at the interface of the metallic diffuser and the fiberglass expansion cone, and/or the existence of high electric fields, and possibly edge effects, in or at the outlet of the expansion cone. The present invention is designed to eliminate the formation of back corona at these interfaces by replacing the diffuser and expansion cone with a unique ionizer assembly outlet section.